# MediaTailor concepts
<a name="what-is-terms"></a>

 Here's an overview of the concepts that are used throughout the *AWS Elemental MediaTailor User Guide*.

# HLS playlist types
<a name="hls-playlist-types"></a>

HTTP Live Streaming (HLS) uses two primary types of playlists: multivariant playlists and media playlists. Understanding the difference between these playlist types is essential for configuring and troubleshooting MediaTailor workflows.

Multivariant playlist  
A multivariant playlist is the top-level index file in an HLS streaming workflow that lists all available renditions of the content. It contains references to media playlists but does not contain any media segments itself. The multivariant playlist allows players to select the most appropriate rendition based on network conditions, device capabilities, or user preferences.  
This playlist type is also known by several other names in various contexts:  
+ Master playlist (deprecated term)
+ Master manifest (common industry term)
+ Primary playlist
+ Main playlist
+ Index file
+ Master M3U8
In MediaTailor workflows, the multivariant playlist is the entry point for playback requests and is where ad personalization begins.  

**Example Multivariant playlist example**  

```
#EXTM3U
#EXT-X-VERSION:3
#EXT-X-INDEPENDENT-SEGMENTS
#EXT-X-STREAM-INF:BANDWIDTH=2665726,AVERAGE-BANDWIDTH=2526299,RESOLUTION=960x540,CODECS="avc1.640029,mp4a.40.2",SUBTITLES="subtitles"
index_1.m3u8
#EXT-X-STREAM-INF:BANDWIDTH=4786126,AVERAGE-BANDWIDTH=4547599,RESOLUTION=1280x720,CODECS="avc1.640029,mp4a.40.2",SUBTITLES="subtitles"
index_2.m3u8
#EXT-X-STREAM-INF:BANDWIDTH=8171787,AVERAGE-BANDWIDTH=7768157,RESOLUTION=1920x1080,CODECS="avc1.640029,mp4a.40.2",SUBTITLES="subtitles"
index_3.m3u8
```

Media playlist  
A media playlist contains the actual media segment information for a specific rendition (quality level) of the content. It includes timing information, segment URLs, and other metadata required for playback of a single rendition. Each media playlist represents a different combination of resolution, bitrate, and other characteristics.  
This playlist type is also known by several other names:  
+ Variant playlist
+ Child manifest (common industry term)
+ Chunklist
+ Media M3U8
+ Rendition playlist
In MediaTailor workflows, media playlists are personalized to include both content segments and ad segments in the proper sequence.  

**Example Media playlist example**  

```
#EXTM3U
#EXT-X-VERSION:6
#EXT-X-TARGETDURATION:6
#EXT-X-MEDIA-SEQUENCE:123456
#EXT-X-DISCONTINUITY-SEQUENCE:0
#EXTINF:6.0,
segment123456.ts
#EXTINF:6.0,
segment123457.ts
#EXT-X-DISCONTINUITY
#EXTINF:5.0,
ad-segment1.ts
#EXTINF:5.0,
ad-segment2.ts
#EXT-X-DISCONTINUITY
#EXTINF:6.0,
segment123458.ts
```

**Note**  
Both multivariant and media playlists are text-based files that use the `.m3u8` extension. The terminology for these files varies across different documentation and contexts, but the fundamental roles remain the same in the HLS streaming architecture.

When configuring MediaTailor, you typically provide the URL to the multivariant playlist in your content origin. MediaTailor then handles the personalization of both the multivariant playlist and the media playlists to insert ads according to your configuration.

For more information about HLS playlist specifications, see the [HTTP Live Streaming RFC](https://datatracker.ietf.org/doc/html/rfc8216).

# DASH manifest types
<a name="dash-manifest-types"></a>

Dynamic Adaptive Streaming over HTTP (DASH) uses a Media Presentation Description (MPD) manifest to deliver streaming content. Understanding the structure and components of DASH manifests is essential for configuring and troubleshooting MediaTailor workflows.

MPD (Media Presentation Description)  
The MPD is the primary manifest file in DASH streaming that describes the structure and availability of media content. It contains information about periods, adaptation sets, representations, and segments that make up the streaming content.  
This manifest type is also known by several other names in various contexts:  
+ DASH manifest
+ DASH MPD
+ Master manifest (when comparing to HLS)
+ Presentation manifest
In MediaTailor workflows, the MPD is the entry point for playback requests and is where ad personalization begins.  

**Example MPD manifest example**  

```
<?xml version="1.0" encoding="UTF-8"?>
<MPD xmlns="urn:mpeg:dash:schema:mpd:2011" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="urn:mpeg:dash:schema:mpd:2011 DASH-MPD.xsd" profiles="urn:mpeg:dash:profile:isoff-live:2011" type="dynamic" minBufferTime="PT5.000S" maxSegmentDuration="PT2.005S" availabilityStartTime="2020-01-01T00:00:00Z" publishTime="2020-01-01T12:30:00Z" minimumUpdatePeriod="PT2.000S" timeShiftBufferDepth="PT5M">
  <Period id="1" start="PT0.000S">
    <AdaptationSet id="1" contentType="video" segmentAlignment="true" bitstreamSwitching="true" frameRate="30000/1001" maxWidth="1920" maxHeight="1080" par="16:9">
      <Representation id="1" mimeType="video/mp4" codecs="avc1.640028" width="1920" height="1080" bandwidth="5000000">
        <SegmentTemplate timescale="90000" initialization="init-stream$RepresentationID$.m4s" media="chunk-stream$RepresentationID$-$Number%05d$.m4s" startNumber="1" duration="180000"/>
      </Representation>
      <Representation id="2" mimeType="video/mp4" codecs="avc1.4d401f" width="1280" height="720" bandwidth="2800000">
        <SegmentTemplate timescale="90000" initialization="init-stream$RepresentationID$.m4s" media="chunk-stream$RepresentationID$-$Number%05d$.m4s" startNumber="1" duration="180000"/>
      </Representation>
      <Representation id="3" mimeType="video/mp4" codecs="avc1.4d401e" width="640" height="360" bandwidth="1100000">
        <SegmentTemplate timescale="90000" initialization="init-stream$RepresentationID$.m4s" media="chunk-stream$RepresentationID$-$Number%05d$.m4s" startNumber="1" duration="180000"/>
      </Representation>
    </AdaptationSet>
    <AdaptationSet id="2" contentType="audio" segmentAlignment="true" lang="eng">
      <Representation id="4" mimeType="audio/mp4" codecs="mp4a.40.2" audioSamplingRate="48000" bandwidth="128000">
        <SegmentTemplate timescale="48000" initialization="init-stream$RepresentationID$.m4s" media="chunk-stream$RepresentationID$-$Number%05d$.m4s" startNumber="1" duration="96000"/>
      </Representation>
    </AdaptationSet>
  </Period>
</MPD>
```

Period  
A Period is a temporal section of a DASH presentation. Each Period contains one or more adaptation sets and represents a span of media time. In ad insertion workflows, separate Periods are typically used to delineate between content and ads.  
This component is also known by several other names:  
+ Content segment
+ Timeline section
+ Program segment
In MediaTailor workflows, Periods are used to separate main content from ad content, with each ad typically represented by its own Period.  

**Example Period example**  

```
<Period id="ad-period-1" start="PT30.000S" duration="PT15.000S">
  <AdaptationSet id="1" contentType="video" segmentAlignment="true" bitstreamSwitching="true" frameRate="30000/1001" maxWidth="1920" maxHeight="1080" par="16:9">
    <Representation id="1" mimeType="video/mp4" codecs="avc1.640028" width="1920" height="1080" bandwidth="5000000">
      <SegmentTemplate timescale="90000" initialization="ad1/init-stream$RepresentationID$.m4s" media="ad1/chunk-stream$RepresentationID$-$Number%05d$.m4s" startNumber="1" duration="180000"/>
    </Representation>
  </AdaptationSet>
</Period>
```

AdaptationSet  
An AdaptationSet groups a set of interchangeable encoded versions of one or several media content components. For example, one AdaptationSet might contain multiple video quality levels, while another might contain multiple audio language options.  
This component is also known as:  
+ Media component group
+ Stream set
+ Track group
In MediaTailor workflows, AdaptationSets are preserved during ad insertion to maintain consistent media types between content and ads.

Representation  
A Representation is a specific encoded version of the media content within an AdaptationSet. Each Representation typically differs in bitrate, resolution, or other encoding parameters, allowing clients to select the most appropriate version based on network conditions and device capabilities.  
This component is also known as:  
+ Rendition (similar to HLS)
+ Quality level
+ Bitrate variant
+ Stream variant
In MediaTailor workflows, Representations in ad Periods are matched as closely as possible to the Representations in content Periods to ensure a smooth viewing experience.

Segment  
A Segment is a unit of media data that can be referenced individually by a URL. Segments contain the actual media content (video, audio, etc.) and are referenced within the MPD. There are two main types of segments in DASH:  
+ **Initialization Segment**: Contains initialization information for a Representation, such as codec parameters and timing information.
+ **Media Segment**: Contains the actual media data for a specific time range within a Representation.
In MediaTailor workflows, segment URLs are often modified to point to the appropriate content or ad media files.

**Note**  
DASH manifests are XML-based files that typically use the `.mpd` extension. The terminology for these files and their components may vary across different documentation and contexts, but the fundamental structure remains the same in the DASH streaming architecture.

When configuring MediaTailor, you provide the URL to the MPD manifest in your content origin. MediaTailor then handles the personalization of the manifest, typically by inserting additional Periods for ads according to your configuration.

For more information about DASH manifest specifications, see the [DASH Industry Forum Implementation Guidelines](https://dashif.org/docs/DASH-IF-IOP-v4.3.pdf).

# Advanced DASH manifest concepts
<a name="dash-manifest-advanced-concepts"></a>

When working with DASH manifests in MediaTailor, understanding the following advanced concepts can help you configure and troubleshoot your streaming workflows more effectively:

Period start time calculation  
In DASH manifests, Period start times are calculated based on the MPD's `availabilityStartTime` attribute and the Period's `start` attribute. For live streams, the start time is relative to the availability start time, while for VOD content, it's typically relative to the beginning of the presentation.  
When MediaTailor inserts ad Periods, it carefully calculates the start times to ensure seamless transitions between content and ads. This calculation takes into account:  
+ The original Period's start time
+ The duration of preceding ad Periods
+ Any time offset specified in the ad decision server response

Preroll timing calculations  
Preroll ads in DASH manifests require special handling because they appear before the main content begins. MediaTailor inserts preroll ads as separate Periods at the beginning of the manifest with appropriate start times and durations.  
For preroll ads, MediaTailor:  
+ Creates a new Period for each preroll ad
+ Sets the first preroll Period's start time to 0
+ Adjusts the start time of the main content Period to account for the total duration of all preroll ads

Live-to-VOD transitions  
DASH manifests can transition from live to VOD (Video on Demand) format when a live stream ends. This transition involves changing the MPD's `type` attribute from `dynamic` to `static` and adjusting other attributes like `timeShiftBufferDepth` and `minimumUpdatePeriod`.  
When MediaTailor processes manifests during live-to-VOD transitions, it ensures that:  
+ Ad markers are preserved in the VOD manifest
+ Period start times are adjusted to maintain proper timing
+ The manifest remains compatible with VOD playback clients

DRM handling  
Digital Rights Management (DRM) information in DASH manifests is typically included in the `ContentProtection` elements within AdaptationSets or Representations. MediaTailor preserves these elements during ad insertion to ensure that content protection remains intact.  
When processing DRM-protected content, MediaTailor:  
+ Maintains all ContentProtection elements from the original manifest
+ Ensures that ad content uses compatible DRM schemes if applicable
+ Preserves any DRM-related attributes and elements throughout the manifest

SCC flags  
Supplemental Content Control (SCC) flags in DASH manifests provide additional information about content characteristics and playback requirements. These flags are typically included as attributes or elements within the MPD structure.  
Common SCC flags that MediaTailor processes include:  
+ Content rating information
+ Accessibility features (closed captions, audio descriptions)
+ Content advisory notices
+ Playback restrictions
MediaTailor preserves these flags during manifest processing to ensure that all content metadata is maintained in the personalized manifest.

Understanding these advanced concepts helps you configure MediaTailor for optimal performance and troubleshoot any issues that may arise in your DASH streaming workflows.

## Ad insertion concepts
<a name="what-is-ssai-terms"></a>

Here's an overview of the concepts that are related to ad insertion.

**Ad avail**  
A specific unit of advertising time within an ad break that can be sold to advertisers. An ad break may contain multiple ad avails. When MediaTailor receives a VAST response from an ad decision server, it fills these avails with personalized ads.

**Ad break**  
The time period during programming when commercials are shown. Ad breaks can occur before content (pre-roll), during content (mid-roll), or after content (post-roll). MediaTailor identifies ad breaks in manifests through markers like SCTE-35.

**Ad decision server (ADS)**  
A server that provides advertising spot specifications based on criteria including current advertising campaigns and viewer preferences. 

**Configuration**  
An object in MediaTailor that you interact with. The configuration holds location information about the origin server and the ad decision server (ADS). The configuration also holds endpoints that provide access points in and out of MediaTailor.

**Dynamic transcoding**  
A process that matches the ad quality and format to the primary video content when content is requested. Dynamic transcoding reduces storage requirements and ensures that playback seamlessly transitions between the ad and video content.

**Manifest manipulation**  
The process of rewriting manifests from the origin server so that the manifests reference the appropriate ad and content fragments. Ads are determined by the VAST response from the ad decision server (ADS). As playback progresses, MediaTailor performs ad insertion or ad replacement into the content stream.

**VAST and VMAP**  
Video Ad Serving Template (VAST) and Video Multiple Ad Playlist (VMAP) are XML responses that the ad decision server sends to ad requests from MediaTailor. The responses dictate what ads MediaTailor inserts in the manifest. VMAP also includes timing for ad breaks and the ad avails within them. For more information about the logic behind MediaTailor ad insertion, see [Understanding AWS Elemental MediaTailor ad insertion behavior](ad-behavior.md). For more information about how MediaTailor works with VAST, see [MediaTailor ad server integration requirements](vast.md).

## Channel assembly concepts
<a name="what-is-ca-terms"></a>

Here's an overview of the concepts that are related to channel assembly.

**Channels**  
A channel assembles your source manifests into a linear stream. Each channel has one or more outputs that contain playback URLs accessed by players. The channel outputs correspond to the package configuration settings you create for your VOD sources. A channel contains a schedule, which determines when VOD sources will play in the channel's stream.

**Package configuration**  
A packager configuration is a representation of your VOD source that contains specific packaged format characteristics. You associate your package configurations with channel outputs to create playback streams for your VOD source's packaged formats, such as HTTP Live Streaming (HLS).

**Schedule**  
Each channel is made up of programs that are arranged into the channel's schedule. The schedule determines what time the programs will play in the channel's linear stream.

**Source locations**  
A source location represents the origin server where your assets are stored. It can be Amazon S3, an HTTP server, a Content Delivery Network (CDN), or a packaging infrastructure such as MediaPackage.

**VOD sources**  
A VOD source represents a single piece of content, such as a movie or an episode of a TV show. You associate VOD sources with programs to add them to your channel's linear stream.

**Audience**  
An audience defines a viewer cohort which can optionally have alternate content. You can define audiences on standard linear channels.